Field of the Invention
The present invention relates to an additive for an acid zinc alloy plating bath, an acid zinc alloy plating bath, and a method for producing a zinc alloy plated article.
Zinc alloy plating herein refers to plating made of zinc and alloy elements, and unavoidable impurities. Such zinc alloy plating may have a content of zinc (% by mass) higher than the content of every other alloy element (% by mass) in the plating, or may have a content of an alloy element higher than the zinc content (% by mass).
Background Art
The plated coatings of zinc alloys, such as a zinc-nickel alloy, a zinc-iron alloy, and a tin-zinc alloy (herein also referred to as “zinc alloy plated coatings”), are widely used for items around us, including machine parts made of steel, such as steel plates, bolts, and nuts for automobiles, to improve their resistance to corrosion, heat, and salt water.
A zinc alloy plated coating is formed by electroplating, or electrolysis performed in a plating bath intended for forming a zinc alloy plated coating (herein also referred to as a “zinc alloy plating bath”), in which a workpiece (an article to be plated) is immersed. Such zinc alloy plating baths can roughly be either alkaline baths (e.g., Japanese Unexamined Patent Application Publication No. 1-298192) or acidic baths (e.g., Japanese Patent No. 4307810). Alkaline baths include cyanide baths and zincate baths, whereas acidic baths include zinc chloride baths and zinc sulfate baths. A bath is selected from such zinc alloy plating baths to suit various conditions including the hardness and the brightness of an intended zinc alloy plated coating, the shape and the size of an article to be plated, and the operating environment.
Among these zinc alloy plating baths, acid zinc alloy plating baths have high current efficiency and thus have high productivity. However, an article plated using an acid zinc alloy plating bath can have its coating thickness or its appearance highly dependent on the current density. An article with a complicated shape can easily have lower coverage or defective appearance.